The 25-year-old full-back originally joined the Saints at the start of this campaign on a season-long loan, but he has now signed a four-and-a-half-year contract.
Bertrand was sent off during Sunday’s 1-0 loss at home to Swansea for a challenge on Modou Barrow and will have to serve a three-match suspension.
But he has generally fitted in very well for the Saints following the sale of last season’s regular left-back, Luke Shaw, making 27 appearances this season and scoring twice.
Bertrand had been at Chelsea since 2005 but spent time on loan at five different clubs before beginning to feature regularly for the Blues.
He made his Premier League debut for the Londoners in April 2011 and played 14 times the following season, incredibly making his Champions League debut in the final against Bayern Munich, which the Blues won on penalties.
Bertrand went on to score his first Chelsea goal in the 2012 Community Shield and was a mainstay of the side that season.
However, his appearances became less frequent following the return to the club of manager Jose Mourinho in the summer of 2013 and he was loaned out to Aston Villa towards the end of last season.
An England regular at age-group levels, Bertrand has two senior caps, having come off the bench against Italy and Ukraine in 2012.